An optical fiber's refractive-index profile is generally described as a relationship of refractive index and the optical fiber's radius. Conventionally, the distance r from the center of the optical fiber is plotted along the abscissa (i.e., the x axis), and the difference between the refractive index at a distance r and the refractive index of the outer cladding of the optical fiber is plotted along the ordinate (i.e., the y axis). The outer cladding, functioning as an optical cladding, typically has a refractive index that is substantially constant. This outer cladding is typically made of pure silica but may also contain one or more dopants.
The refractive-index profile may have a “step” profile, a “trapezoidal” profile, a “parabolic” profile (e.g., an “alpha” profile), or a “triangular” profile, which can be graphically depicted as a step, trapezoidal, parabolic, or triangular shape, respectively. These curves are generally representative of the theoretical or design profile of the optical fiber. Constraints associated with optical-fiber fabrication may lead in practice to a profile that is perceptibly different.
An optical fiber conventionally includes an optical core, which has the function of transmitting and optionally amplifying an optical signal. A conventional optical fiber also typically includes an optical cladding, which confines the optical signal in the core. For this purpose, the refractive index of the core nc is typically greater than the refractive index of the cladding ng (i.e., nc>ng). As will be understood by those having ordinary skill in the art, the propagation of an optical signal in a single-mode optical fiber includes a fundamental mode, typically denoted LP01, which is guided in the core, and secondary modes, which are guided over a certain distance in the core and the optical cladding.
Single-mode optical fibers (SMFs) with step-index profile are often used within optical-fiber transmission systems. Such optical fibers typically possess a chromatic dispersion and a chromatic-dispersion slope that comply with specific telecommunications standards.
Conventionally, so-called “standard” single-mode fibers (SSMFs) are used for land-based transmission systems. To facilitate compatibility between optical systems from different manufacturers, the International Telecommunication Union (ITU) has defined a standard reference ITU-T G.652 with which a standard optical transmission fiber (i.e., a standard single-mode fiber or SSMF) should comply. The ITU-T G.652 recommendations (November 2009) and each of its attributes (i.e., A, B, C, and D) are hereby incorporated by reference.
Among other recommendations for a transmission fiber, the ITU-T G.652 standard recommends (i) a mode field diameter (MFD) with a nominal value (e.g., a nominal mode field diameter) of between 8.6 microns (μm) and 9.5 microns and a tolerance of ±0.6 micron at a wavelength of 1310 nanometers (nm), (ii) a maximum cable cut-off wavelength (λCC) of 1260 nanometers (nm), (iii) a zero-dispersion wavelength (ZDW) of between 1300 nanometers and 1324 nanometers, and (iv) a maximum zero-dispersion slope (ZDS) of 0.092 picoseconds per square nanometer kilometer (ps/(nm2·km)) (i.e., the chromatic-dispersion slope at the zero-chromatic-dispersion wavelength is 0.092 ps/(nm2·km) or less).
The cable cut-off wavelength is conventionally measured as being the wavelength at which the optical signal is no longer single mode after propagating over 22 meters in the optical fiber, as defined by subcommittee 86A of the International Electrotechnical Commission (IEC) in standard IEC 60793-1-44. The IEC 60793-1-44 is hereby incorporated by reference in its entirety.
In most circumstances, the secondary mode that best withstands bending losses is the LP11 mode. The cable cut-off wavelength is thus the wavelength from which the LP11 mode is sufficiently attenuated after propagating for 22 meters in an optical fiber. The method proposed by the ITU-T G.652 standard considers that the optical signal is single mode as long as the attenuation of the LP11 mode is greater than or equal to 19.3 decibels (dB). According to the recommendations of IEC subcommittee 86A in standard IEC 60793-1-44, the cable cut-off wavelength is determined by imparting two loops having a radius of 40 millimeters (mm) in the optical fiber, while arranging the remainder of the optical fiber (i.e., 21.5 meters of optical fiber) on a mandrel having a radius of 140 millimeters.
For a given fiber, a MAC value is defined as being the radius of the mode diameter of the optical fiber at 1550 nanometers over the fiber cut-off wavelength (λC) (i.e., “effective cut-off wavelength”). The fiber cut-off wavelength is conventionally measured as the wavelength at which the optical signal is no longer single mode after propagating over two meters of fiber, as defined by IEC subcommittee 86A in the standard IEC 60793-1-44. The MAC value constitutes a parameter for assessing the performance of the optical fiber, particularly for finding a compromise between the mode field diameter, the fiber cut-off wavelength, and bending losses.
Commonly owned European Patent No. 2,116,878 (and its counterpart U.S. Patent Application Publication No. 2009/0279835), commonly owned European Patent No. 2,116,877 (and its counterpart U.S. Pat. No. 7,889,960), commonly owned European Patent No. 1,930,753 (and its counterpart U.S. Pat. No. 7,555,186), commonly owned European Patent No. 1,845,399 (and its counterpart U.S. Pat. No. 7,587,111), and commonly owned European Patent No. 1,785,754 (and its counterpart U.S. Pat. No. 7,623,747), each of which is hereby incorporated by reference in its entirety, propose single-mode optical fibers having limited bending losses. European Patent Nos. 1,845,399 and 1,785,754 have experimental results showing a relationship between (i) the MAC value at a wavelength of 1550 nanometers and (ii) bending losses at a wavelength of 1625 nanometers with a radius of curvature of 15 millimeters in a step-index standard single-mode fiber. These documents establish that the MAC value has an influence on optical-fiber bending losses. Moreover, these documents demonstrate that bending losses can be reduced by reducing the MAC value.
Unfortunately, reducing the MAC value by reducing the mode diameter and/or by increasing the fiber cut-off wavelength (λC) can lead to noncompliance with the ITU-T G.652 recommendations, thereby making an optical fiber commercially incompatible with certain transmission systems.
Accordingly, reducing bending losses while remaining compliant with industry recommendations constitutes a genuine challenge for fiber applications for use in various optical-fiber systems (e.g., fiber to the home (FTTH)).
The ITU (International Telecommunication Union) has also defined standards relating to bend-insensitive optical fibers, in particular the ITU-T G.657.A standards (e.g., the ITU-T G.657.A1 (November 2009) and the ITU-T G.657.A2 (November 2009) subcategories) and ITU-T G.657.B standards (e.g., the ITU-T G.657.B2 (November 2009) and the ITU-T G.657.B3 (November 2009) subcategories). The ITU-T G.657.A recommendations impose bending loss limits but seek above all to maintain compatibility with the ITU-T G.652 recommendations (e.g., the ITU-T G.652.D recommendations), particularly with respect to mode field diameter and chromatic dispersion. In contrast, the ITU-T G.657.B recommendations do not impose compatibility with ITU-T G.652 recommendations but impose stricter limits on bending losses than those imposed by the ITU-T G.657.A1 recommendations.
Aforementioned European Patent Nos. 1,845,399 and 1,785,754 propose fiber profiles that facilitate limited bending losses typically satisfying ITU-T G.657 A/B recommendations.
U.S. Pat. No. 7,187,833, which is hereby incorporated by reference in its entirety, proposes parabolic profiles for optical fibers that include a trench, seeking to obtain a large effective area and low attenuation as a function of distance. Nevertheless, U.S. Pat. No. 7,187,833 does not seek to reduce bending losses. Moreover, none of the profiles it proposes comply with any of the ITU-T G.657.A2/B2/B3 recommendations.
Table 1 (below) gives the maximum acceptable macrobending loss for the ITU-T G.652.D and G.657.A1/A2/B2/B3 recommendations for various radii of curvature at wavelengths of 1550 nanometers and 1625 nanometers.
TABLE 1radiuswavelengthmaximum acceptable macrobending losses (dB)(mm)turns(nm)G.652.DG.657.A1G.657.A2G.657.B2G.657.B33010015503010016250.1151015500.250.031510162510.110115500.750.10.0310116251.50.20.17.5115500.50.087.51162510.255115500.155116250.45
For the ITU-T G.652 and G.657.A/B recommendations, Table 2 (below) depicts (i) the nominal value ranges and tolerances associated with the mode field diameter, (ii) the maximum values for the cable cut-off wavelength (λCC), and (iii) the values for the chromatic-dispersion parameters. The chromatic-dispersion parameters λ0min and λ0max designate the minimum and maximum zero-chromatic-dispersion wavelengths (ZDW), respectively. The parameter S0max designates the maximum value for the zero-chromatic-dispersion slope (ZDS).
TABLE 2G.652.DG.657.AG.657.BNominal MFD8.6-9.58.6-9.56.3-9.5(@ 1310 nm) (μm)Tolerance (μm)±0.6±0.4±0.4Maximum cable126012601260cut-off wavelength(nm)λ0 min (nm)13001300λ0 max (nm)13241324s0 max (nm)0.0920.092
Various refractive-index profiles have been proposed to comply with the criteria of the ITU-T G.657.A/B recommendations.
For example, optical fibers having a simple step-index profile have been proposed. These step-index profiles have MAC values selected to ensure conformity with the bending loss parameters of the ITU-T G.657.A1 recommendations. Such an optical fiber is sold by Draka under the trademark BendBright®. Nevertheless, when these profiles comply with the ITU-T G.657.B2 recommendations, they do not necessarily comply with the ITU-T G.657.A or G.652 recommendations because of a small mode field diameter.
Furthermore, optical fibers have been proposed having a step-index profile with a trench in the cladding so as to provide conformity with the ITU-T G.657.A2/B2 recommendations (e.g., fiber sold by Draka under the trademark BendBright-XS®) or the ITU-T G.657.B3 recommendations (e.g., fiber sold by the Applicant under the trademark BendBright-Elite®). By way of example, the trench may be made by adding dopants (e.g., fluorine or boron) for reducing the refractive index. Solutions of this kind are known as solid-trench-assisted (STA). The trench may also be obtained by incorporating holes (i.e., hole-assisted (HA)) or bubbles (i.e., bubble-assisted (BA)).
Nevertheless, those present-day solutions require strict control over the MAC value, and consequently over the mode field diameter and the cut-off wavelength, in order to comply with all of the constraints of the ITU-T G.657 and G.652 recommendations, while also maintaining good yields, in particular by minimizing the production of noncompliant fibers.